1. Field of the Invention
This invention relates to metal frame window assemblies and more particularly to the lower rail of an upper sash and the upper rail of a lower sash each of which has an interlock channel which overlaps the other interlock channel. Similarly, this invention also relates to metal door assemblies and more particularly to the inner stile of an exterior panel and an inner stile of an interior panel each of which has an interlock channel which overlaps the other interlock channel.
2. Description of the Related Art.
The invention particularly relates to thermally insulated windows or doors and similar panel assemblies for installation in an opening of an enclosed wall exposed to different temperature conditions at the opposite sides thereof.
The relatively high thermal conductivity of metals, such as aluminum or alloys thereof, is probably the most undesirable property limiting their substitution for wood and other less conductive non-metallic structural materials. The effect of this metal characteristic is best seen in windows or doors or other panel assemblies employing metal framing in direct conductive relationship with air at interior and exterior ambient temperatures giving rise to the generation and presence of frost and moisture condensation on the interior surfaces of the framing, especially in climates where extreme temperature differentials prevail. When a panel assembly has a metal outer frame fixed in the rough opening of a wall, and a sash movably mounted in such fixed outer frame with a metal frame peripherally embracing a glass or other panel, the thermal conductivity problem exists with respect to the metal sash frame as well as the metal outer or fixed frame.
A wide variety of solutions to overcome the deleterious high thermal conductivity of metal framing for windows, doors, and similar panel assemblies have been proposed with varying degrees of success. Most of the related art constructions have included some type of thermal break or insultaing barrier installed in an appropriate part of the outer metal frame and also in an appropriate part of the sash frame. Such constructions are often complex and difficult to fabricate as well as being relatively expensive. However, a problem in the related art is that no thermal break or insulating barrier is known that operates to prevent heat conduction between the upper sash and the lower sash or side-by-side panels.